Backend [Unit Test] db/config.go CheckUser, & RolesCheck

[Fog3211]

Describe your changes

Backend [Unit Test] db/config.go CheckUser, & RolesCheck

Issue ticket number and link

closes #1562

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist before requesting a review

• I have performed a self-review of my code
• I have tested on Chrome and Firefox
• I have tested on a mobile device
• I have provided a screenshot or recording of changes in my PR if there were updates to the frontend

[Fog3211]

@elraphty @ecurrencyhodler Sorry I only finish test for CheckUser and RolesCheck func, can't finish test for UserHasManageBountyRoles, UserHasAccess func( I tried many ways but all failed 😭， like this)

When I use the global variable DB, a null pointer error will be reported.
When I try to introduce mockDB, I will be prompted that I cannot reference circularly.
When I implement a mock DB myself, I will be prompted that the type is incorrect.